As conventionally known, a car (vehicle) is mounted with an ultrasonic sensor including an ultrasonic oscillator bonded to a substrate. The ultrasonic sensor transmits an ultrasonic wave from an element capable of transmitting and receiving ultrasonic waves. The ultrasonic sensor uses the element to receive the ultrasonic wave reflected on an object to be detected. In this manner, the ultrasonic sensor measures a position of an object around the car, a distance between the object and the car, and a two-dimensional or three-dimensional shape of the object.
In a known ultrasonic sensor (Patent Document 1), for example, a substrate is provided at a tip of a cylindrical aluminum case mounted on a vehicle. The substrate is directly mounted with a piezoelectric ultrasonic oscillator for detecting an ultrasonic wave. The substrate is vibrated to transmit and receive an ultrasonic wave.                Patent Document 1: JP-2002-58097 A        
This type of ultrasonic sensor is mounted at a position easily identifiable from the outside and needs to be miniaturized for aesthetic purposes. However, miniaturizing the ultrasonic sensor degrades sensor characteristics due to an increased resonance frequency, increased signal attenuation, or degraded directivity. Decreasing the resonance frequency just needs to decrease the rigidity of a substrate where the ultrasonic oscillator is to be bonded. Measures of decreasing the substrate rigidity include the use of a substrate with a low Young's modulus, e.g., a resin substrate.
An ultrasonic sensor 110 is formed using a resin substrate. As shown in FIGS. 10A and 10B, the ultrasonic sensor 110 includes a resin substrate 131 and an ultrasonic oscillator 121 formed by sandwiching a piezoelectric substance 121c between an upper electrode 121a a lower electrode 121b. A lead wire 128 is soldered to the upper electrode 121a. A lead wire 125 is soldered to an electrode 123 and electrically coupled with the lower electrode 121b via a conductive adhesive 122.
The ultrasonic oscillator 121 is wired by soldering lead wires to the electrodes. Soldering becomes difficult on the ultrasonic oscillator 121 miniaturized to a square millimeter or smaller. It is preferable to change the wiring method from soldering to wire bonding. Since the resin substrate 131 shows a less heat resistance, however, it cannot bear thermocompression bonding that requires a high temperature of 300 degrees or higher. Since the resin substrate 131 is less rigid, thermosonic bonding (ultrasonic thermocompression bonding) cannot generate a proper frictional heat and is therefore inapplicable. The resin substrate is effective for ultrasonic sensor miniaturization but has been incapable of wiring using the wire bonding.